To support the increased diversity of the future services to be provided by 5G networks, a recently waveform configuration known as filtered-OFDM (filtered Orthogonal Frequency Division Multiplex) has been proposed.
Filtered-OFDM is said to allow a flexible time-frequency arrangement/allocation as opposed to the uniform distribution required by OFDM in 4G long-term evolution networks (LTE). Filtered-OFDM splits up the bandwidth assigned to a given wireless operator into several sub-bands. Further, in each sub-band, a conventional OFDM signal is tailored to suit the needs of a given type of service in accordance with associated channel operating parameters such as the subcarrier spacing, the cyclic prefix length and the transmission time interval.
In other words, each stream of OFDM symbols associated to a given sub-band is isolated and potentially non-orthogonal to other OFDM symbols associated to other sub-bands. The forgoing also means that where different operating parameters are used by OFDM signals of adjacent sub-bands, subcarrier orthogonality between such OFDM signals may generally be broken, thereby generating interferences between adjacent sub-bands.
Such inter sub-band interference is dealt with in filtered-OFDM by using a band-limited filter on each stream of OFDM symbols of each sub-band and which is configured to suppress, or at least reduce below a target level, the out-of-band (OOB) emissions that may exist between adjacent sub-bands.
However, although the filtering approach used in filtered-OFDM provides a high level of out-of-band rejection close to the modulated subcarriers for a given stream of OFDM symbols, it also introduces inter-carrier interference (ICI) which is known to degrade the bit error rate (BER) at the receiver level. Indeed, in filtered-OFDM, filtering is applied on all subcarriers of a given OFDM symbol.
This is not appropriate, as BER is a primary concern in the design of many digital communication systems. Therefore, it would be desirable to have a waveform configuration as effective as filtered-OFDM in terms of OOB rejection, while at the same time being able to minimize the ICI.